arm: mvebu: Move mvebu-common into mach-mvebu

Now that the mach-mvebu directory exists and is used by Armada XP we can
move the mvebu-common files into this directory as well.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Kevin Smith <kevin.smith@elecsyscorp.com>
Tested-by: Dirk Eibach <dirk.eibach@gdsys.cc>